When thread cutting the tool is applied to the bore in the workpiece with a high feed force in order to achieve as immediate a commencement of cutting as possible. A resilient length compensation under compression prevents impermissible shock loads. Later in the thread cutting process a feed is used which is somewhat smaller than the pitch of the thread. The difference is compensated for by a resilient length compensation under tension. The latter is also effective if the tool runs on after the rotary drive and the feed have been switched off. A coupling which permits the tool holding fixture to perform radial movements transverse to the axis of rotation compensates for alignment errors between the tool and the bore.
It has been found in practice that the spring system, which compensates for differences in length of the holder and returns the axially movable portions of the holder into a neutral position, is subject to such high loads that it rapidly fatigues. This applies particularly to the length compensation under compression. In this case a fatigue of the spring device leads to the feed force, which is necessary for the beginning of cutting, building up ever later. An increasing proportion of the pre-programmed feed is thus used up before the tool penetrates into the bore. The consequence is that the depth of the thread does not achieve the necessary and pre-programmed dimension. In order to counteract this disadvantage a frequent readjustment of the programming is necessary to compensate for the spring fatigue.